MOSCOW/BRASILIA, Dec 13 (Reuters) - Brazil could end up increasing beef exports to Russia, its biggest buyer, after banning a controversial feed additive that promotes muscle growth in animals such as pigs and cattle, Russia’s food safety watchdog said on Thursday.

Russia stepped up tests on U.S. and Canadian meat imports for traces of the additive, a beta blocker called ractopamine, on Monday, and demanded both nations certify their meat as ractopamine-free. U.S. exports alone to Russia are worth about $500 million.

Ractopamine is in a class of drugs known as beta inhibitors or blockers that counteract the effects of adrenaline on the nervous system and slow the heart rate. In livestock, it promotes muscle gain.

Brazil banned the use of ractopamine on Nov. 12 and eventually plans to have a split system in place enabling it to produce ractopamine-free meat for some buyers like Russia, and use it in meat for other destinations.

Russia’s food safety watchdog, Rosselkhoznadzor, said it was satisfied with Brazil’s assurances and said Brazil may manage to increase its current share of Russian beef imports, now around 43 percent, as a result. An increase would likely come at the expense of purchases of U.S. and Canadian meats while the testing and certification issue is resolved.

“Everything is alright with Brazilian meat. They guaranteed that meat coming to Russia will be ractopamine free,” Rosselkhoznadzor spokesman Alexei Alekseenko told Reuters.

“It is a very interesting step because they will be ready to keep their stake of the Russian market and even to increase it,” Alekseenko said.

Russia imported 1.25 million tonnes of red meat worth $4.47 billion from non-CIS countries in 2011, excluding offal, according to official customs data.

Russia has denied that political motives lie behind its tighter restrictions on North American meats, saying its decision was based purely on its food safety standards.

Last Friday, the same day that the U.S. Senate voted on a measure to “name and shame” human rights violators as part of a bill expanding trade with Russia, Russia demanded that U.S. and Canadian meat imports be certified to be free of the feed additive ractopamine.

Ironically, at a time when Russia sees the possibility of more Brazilian beef imports, it is still mulling imposing restrictions on them after a case of atypical bovine spongiform encephalopathy, commonly known as mad cow disease, in an animal that died of other causes in southern Brazil in 2010.

The case is believed to have been an “atypical” one arising from a genetic mutation more common in elderly cattle that can spark off the disease spontaneously, unlike cases in Britain in the 1980s and 1990s caused by contaminated feed.

The animal was buried on the farm where it died and never entered the food chain.

Alekseenko said Russia would finish its investigation into the Brazilian case soon. “I think the results will come pretty soon and then we will be able to decide on what to do.”

Japan announced it was suspending purchases of Brazilian beef on Monday and has requested further information about the animal from Brazilian authorities. South Africa said on Thursday it was restricting Brazilian beef purchases. Both are small buyers of Brazil’s meat.